Although incandescent lamps have traditionally been used for larger-scale lighting, signaling and imaging functions, such applications are increasingly making use of light-emitting diodes owing to their long-life, high brightness and relatively low power consumption. Their response time from off to on and back is also improved over incandescent lamps, providing additional functionality.
A light-emitting diode (LED) emits a light when electrons from an N-region and holes from a P-region are moved to a PN junction section and are recoupled in a PN junction semiconductor when a forward voltage is applied. Since the energy released when the free electrons are coupled is radiated as light, the light emitted from the LED is light in a narrow wavelength range, i.e. light of a single color such as red or blue.
A so-called “full-color” LED device can generate white light by using a combination of a red light-emitting element for emitting red light, a blue light-emitting element for emitting blue light and a green light-emitting element for emitting green light. Research has been undertaken to develop an LED device capable of emitting white color using one kind of light-emitting element. Such an LED device is called a white LED device and is distinguishable from the full-color LED device. Using a white LED device enables a back light or the like to be driven by a simple electric circuit without requiring an inverter circuit and the like, and reduces power consumption. This presents advantages such as reducing the outer configuration of the driving circuit and eliminating electromagnetic noise.
Since a white LED device must generate white light by using only one kind of light-emitting element (single-color), it has been the general practice to use a single-color light-emitting element in combination with a phosphor that can convert the wavelength of light emitted from the light-emitting element to emit light of another color. For example, white LED devices which have a layer containing YAG (yttrium aluminate) phosphor provided on a blue light-emitting element are commercially available. The YAG phosphor generally emits light in the green to yellow region, depending on the kind of activating element in the YAG.
Owing to the increased flexibility and affordability of LEDs able to emit light across various frequencies, thereby enabling full-color display, modem electronic displays increasingly employ LEDs for larger-scale imaging and display applications.